279 research outputs found
Energetic particle phase space densities at Saturn: Cassini observations and interpretations
No full textSaturn's magnetosphere has been studied extensively by the Cassini spacecraft during the last 6 years. We present mission-averaged energetic proton and electron measurements obtained by the MIMI/LEMMS instrument onboard Cassini in an energy range from several 10 keV to several 10 MeV separated by equatorial pitch angle. We discuss the resulting radial profiles and energy spectra. The measured intensities are converted to phase space densities. The distribution of energetic particles is governed by a large variety of processes. For instance, moons absorb energetic particles, creating macrosignatures or microsignatures. We have found that the moon Rhea is partly responsible for a change in gradient of electron phase space densities. We show that, in contrast to larger distances, the particle distribution for L < 8 is not driven by radial diffusion alone. There, the particle profiles are significantly modified due to Saturn's Neutral Torus, plasma environment, E ring, injection events, and cosmic ray albedo neutron decay. Large parts of our analysis are focused near L = 7. There, protons are lost within the Neutral Torus and not the E ring. For electrons, we find that these two losses are of comparable rate but have discovered that neither process is the dominant driver of loss. We point out that intensity measured by a energy channel, such as in a particle instrument, can actually increase in the region of ring and torus instead of decrease. The importance of injection events is shown to be at least of similar importance as radial diffusion.</p
A radiation belt of energetic protons located between Saturn and its rings
No full textSaturn has a sufficiently strong dipole magnetic field to trap high-energy charged particles and form radiation belts, which have been observed outside its rings. Whether stable radiation belts exist near the planet and inward of the rings was previously unknown. The Cassini spacecraft’s Magnetosphere Imaging Instrument obtained measurements of a radiation belt that lies just above Saturn’s dense atmosphere and is decoupled from the rest of the magnetosphere by the planet’s A- to C-rings. The belt extends across the D-ring and comprises protons produced through cosmic ray albedo neutron decay and multiple charge-exchange reactions. These protons are lost to atmospheric neutrals and D-ring dust. Strong proton depletions that map onto features on the D-ring indicate a highly structured and diverse dust environment near Saturn
Discovery of a transient radiation belt at Saturn
No full textRadiation belts have been detected in situ at five planets. Only at Earth however has any variability in their intensity been heretofore observed, in indirect response to solar eruptions and high altitude nuclear explosions. The Cassini spacecraft's MIMI/LEMMS instrument has now detected systematic radiation belt variability elsewhere. We report three sudden increases in energetic ion intensity around Saturn, in the vicinity of the moons Dione and Tethys, each lasting for several weeks, in response to interplanetary events caused by solar eruptions. However, the intensifications, which could create temporary satellite atmospheres at the aforementioned moons, were sharply restricted outside the orbit of Tethys. Unlike Earth, Saturn has almost unchanging inner ion radiation belts: due to Saturn's near-symmetrical magnetic field, Tethys and Dione inhibit inward radial transport of energetic ions, shielding the planet's main, inner radiation belt from solar wind influences
The spatial variations in the surface properties due to the interaction with the planet's magnetosphere: Dione and Rhea
No full textMapping of the variations of the physical and chemical surface properties on icy satellites (Stephan et al., 2009a,b,c; Jaumann et al., 2008) has shown that the knowledge of the spatial variations in the surface properties are essential to resolve the origin of the specific surface compounds or explain the spatial variations in their physical characteristics. The interaction between the satellite surfaces and their planetary environment is different for each satellite depending on its location within the planetary system (Paranicas et al., 1990). In order to understand the association of specific surface compounds of the Jovian and the Saturnian satellites to the interaction with the planet's magnetosphere a detailed mapping of the location and spatial extension of their spectral properties with respect to the satellites specific location within the planet's magnetosphere was started. Results will be presented for the Saturnian satellites Dione in comparison to its outer neighbour Rhea
Sources, sinks and transport of energetic electrons near Saturn’s main rings
No full textThe inner boundary of Saturn's electron radiation belts, near the planet's A‐ring (∼2.27 Rs), is studied using Cassini's Proximal orbit measurements. We find that variable convective flows transport energetic electrons to the A‐ring, which absorbs them instantaneously, forming the inner belt boundary. These flows are also responsible for a variable and longitudinally asymmetric boundary configuration. Pre‐noon, the boundary oscillates towards and away from the A‐ring with a two‐week period. Post‐noon, it maps persistently near the F‐ring (∼2.32 Rs) and coexists with localized MeV electron intensity enhancements (microbelts). We propose that the microbelts contain electrons in drift resonance with corotation, trapped in local‐time confined trajectories which result from the aforementioned convective flows. The microbelts' collocation with the F‐ring implies either a local, secondary electron production due to Galactic Cosmic Ray collisions with F‐ring dust, or an enhanced resonant electron trapping due to an electrodynamic interaction between the F‐ring and Saturn's magnetosphere
Inflow Speed Analysis of Interchange Injections in Saturn's Magnetosphere
No full textDuring its more than 13 years in orbit, the Cassini spacecraft detected a large number of plasma and energetic charged particle injections in Saturn's inner magnetosphere. In the corotating frame of the planet, the plasma contained within an injection moves radially inward with the component particles gaining energy. The highest energy particles in the injection experience stronger gradient‐curvature drifts in the longitudinal direction and can drift out of the main body of the injection. We have used these drift‐out effects to estimate the inflow speed of 19 injections by surveying cases from the available plasma data. We find that the average inflow speed from our sample is 22 km/s, and the values are well distributed between 0 and 50 km/s, with a few higher estimates. We have also computed the radial travel distance of interchange events and found that these are typically one to two Saturn radii. We discuss the implications of these quantifications on our understanding of transport
Identifying fast plasma injections in data from Saturn
No full textIn Paranicas et al. (2020), we reported on a method to estimate the inflow speed of interchange injections in Saturn's magnetosphere. The procedure relies on phase space density conservation and mapping and an estimate of the size of the flux decrease along one edge of the injection. Here we describe modifications to the method. We have applied our new technique to an existing list of injections, presenting only those injections with inflow speeds greater than 20 km/s, defined as "fast" injections here. We find at least 20% of the events from the list can be considered fast. Faster injections are more effective in energizing charged particles as the injection moves planetward. This is because shorter transit time limits the number of particles that can drift longitudinally out of the injection
Heliospheric conditions at Saturn during Cassini's Ring-Grazing and Proximal Orbits
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Ion composition in interchange injection events in Saturn's magnetosphere
No full textInterchange injection events are commonly observed by the Cassini spacecraft in the region between about 6 and 12 Rs (1 Rs = 60,268 km) and even frequently beyond. In this study, 13 examples of interchange injection events are identified in Cassini/Cassini Plasma Spectrometer data under special conditions such that time-of-flight (TOF) mass spectra could be obtained from entirely within the events. Using the TOF data to separate the main ion species H+, H2+, and W+, approximate densities of each species are calculated under the assumption that all distributions were isotropic. The light-ion density ratios, H2+/H+, in the injection events are not discernibly different from those ratios in control intervals from the ambient plasma. However, the water-group ratio, W+/H+, is significantly lower than ambient. The comparison of the measured density ratios with the range of values observed throughout Saturn's magnetosphere indicates that the values of W+/H+ that are as low as those observed within the injection events are found primarily beyond L~14 (where L is the equatorial crossing distance, in Saturn radius, of a dipole field line), indicating that the injection events are delivering plasma from the outer magnetosphere at times traveling at least 6 Rs.</p
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